Conventional printed circuit boards are discrete workpieces which bear a certain circuit configuration on a rigid substrate, if appropriate on both sides. The circuit configurations on the opposite sides can be electrically connected to each other via through-holes, the lateral surfaces of which have been provided with a metallic coating by electroplating. Similarly constructed are the so-called multilayer inner coatings. These are likewise discrete items, which however comprise a flexible substrate and, stacked one on top of the other, are connected to one another to form a three-dimensional circuit. There is a development towards replacing these discrete circuit boards or multilayer inner coatings by thin plastic films into which the required through-holes are made by special dry plasma-etching processes. These processes are not only considerably less expensive than mechanical drilling, they additionally allow the production of through-holes of diameters smaller than can be achieved by mechanical means. The through-bores obtained have, moreover, an extremely favourable aspect ratio, that is the ratio of length to diameter of the through-hole. The handling of the plastic film is also much easier than that of the discrete circuit boards. This applies all the more so in comparison with discrete pieces of film, which are extremely difficult to process.
The plastic films are provided on the surfaces to be electroplated with a conductive coating. On the planar "main surfaces", this conductive coating is generally composed of metal, whereas the lateral surfaces of any existing through-holes are often made conductive by a suitable polymer layer. When "plastic film" is mentioned hereinafter, it is always intended to be a simple way of referring to the coated plastic film.
Known apparatus for the electroplating of such thin plastic films, which are generally produced from polyimide, operate such that they pass the plastic film in the electrolyte repeatedly up and down in a serpentine manner between lower and upper deflecting rolls. It is intended by this to keep the dimensions of the electroplating apparatus small, while maintaining a long path of movement of the plastic film in the bath. However, such apparatus are very complex in their construction; the repeated deflection about tile upper and lower deflecting rolls can mechanically impair the metal layers as they are formed.
The present invention seeks to provide apparatus for the electroplating of thin plastic films that it is simple in mechanical construction, without requiring greater dimensions and without impairing the plating operation itself.
Accordingly, the invention provides apparatus for the electroplating of thin plastic films, provided on one or both sides with a conductive coating, which comprises
a) at least one assigned supply reel for the plastic film to be electroplated; PA1 b) at least one assigned supply reel for the electroplated plastic film; PA1 c) a conveying device which conveys the plastic film continuously from one supply reel to the other; PA1 d) at least one chamber which can be charged with electrolyte, which chamber lies between the supply reels and in which there is located in the vicinity of the path of movement of the plastic film at least one anode which is electrically connected to the one pole of an electroplating current source; PA1 e) at least one bonding device which is electrically connected to the other pole of the electroplating current source and establishes contact with the moving plastic film, PA1 a) a main cooler, by which the electrolyte located in the sump is kept below a first preselectable temperature; PA1 b) at least one auxiliary cooler, by which the electrolyte taken from the sump can be cooled on the way to the chamber which can be charged with electrolyte and which keeps this electrolyte at a second preselectable temperature, which is lower than the first temperature. PA1 a) a supply of metallic copper; PA1 b) a device by which part of the electrolyte can be enriched with oxygen and can be fed to the metallic copper.
in which the conveying device is set up in such a way that the plastic film runs horizontally in the entire region of the chamber which can be charged with electrolyte.
According to the invention, the plastic film in the electrolyte bath can be kept horizontal during the electrolysis operation. The lengthening of the path that has been achieved previously by passing the film up and down in a serpentine form, is dispensed with completely. Within the same dimensions in the direction of movement of the plastic film, the apparatus according to the invention provides a path for the film in the electrolyte is shorter than in the case of the known apparatus; however, this is compensated by the fact that the geometrical relationships in the way in which the plastic film is passed through the electrolyte according to the invention are considerably more favourable. In particular, the flowing of electrolyte onto the plastic film can be designed such that no depletion and concentration effects occur.
In this case, it is known per se from DE-A-3624481 or DE-A-3236545 to electroplate discrete circuit boards of a conventional type running horizontally through. In the case of these discrete circuit boards however, there is no possibility of upward and downward movement in a serpentine form in the electrolyte bath with a tub of design which is to some extent reasonable in its complexity.
The bonding device should in this case be arranged outside the chamber which can be charged with electrolyte. This is a further advantage of the endless plastic film in comparison with discrete circuit boards: the bonding does not have to take place in the electrolyte itself, where metal would likewise be deposited on the bonding device. This represents a considerable problem in the case of the known apparatus for producing discrete circuit boards.
Preferably, the chamber for electrolyte is subdivided into a plurality of electroplating chambers which are arranged one behind the other, in the direction of movement of the plastic film. This is of significance in particular if, as mentioned above, the bonding is to take place outside the electrolyte. However, the spacing between two bonding devices following one behind the other in the direction of movement of the plastic film should preferably not be too great, so that that voltage drops within the plastic film do not result in non-uniform electroplating.
It is therefore preferable for the bonding devices to be arranged upstream of, between and downstream of the electroplating chambers. The length of the individual electroplating chambers is then governed by the permissible voltage drop within the plastic film.
Preferably, at the inlet and at the outlet of each electroplating chamber there is arranged at least one pair of squeezing rolls, which serve as the only conveying device, that is to say that no additional conveying devices, such as transport rollers, are provided between the pairs of squeezing rolls at the inlet and at the outlet. Rather, the plastic film freely spans the entire electroplating chamber between the pair of squeezing rolls at the inlet and the pair of squeezing rolls at the outlet. This type of construction is particularly inexpensive. In addition, the risk of distortions of the plastic film when running through the entire apparatus drops all the more the smaller the number of places at which the plastic film is held firm.
It goes without saying that the plastic film must not bend within the electroplating chamber to such an extent that one of the neighbouring anodes is touched. In general (as long as the aspect mentioned above of the voltage drop docks not apply), the electroplating chambers can be kept all greater in length the better the plastic film is kept taut. Various possibilities are available for this. For example, at the inlet and at the outlet of each electroplating chamber there may be in each case two pairs of squeezing rolls arranged in such a way that they converge toward each other in the direction of movement of the plastic film and enclose an obtuse angle. If these pairs of squeezing rolls are driven, they draw the plastic film not only in the desired conveying direction but also at the same time perpendicularly to the latter outward, which effects a transverse tautening.
The obtuse angle may be from about 120.degree. to about 190.degree., preferably about 150.degree..
If, as often happens in the case of discrete circuit boards, the bonding device is formed by lateral pairs of contact rollers, a similar effect can also be accomplished by the axes of pairs of contact rollers lying opposite one another on both sides of the plastic film converging toward each other in the direction of movement of the plastic film and enclosing an obtuse angle.
A tautening of the plastic film in the direction of movement can be achieved by the circumferential speed of the pairs of squeezing rolls increasing in the direction of movement of the plastic film. The pairs of squeezing rolls following in the direction of movement thus attempt to draw the plastic film away slightly more quickly all the time than it is being supplied by the upstream pairs of squeezing rolls. In this way, there is always some slip, which in the simplest case takes place between the pairs of squeezing rolls and the plastic film itself.
The increasing circumferential speed of the pairs of squeezing rolls can be achieved again in various ways. For example, with a view to the drive technique, it can be preferred for the diameter of the pairs of squeezing rolls to increase in the direction of movement of the plastic film. The rotational speed of all the pairs of squeezing rolls in the apparatus can then be the same; all the pairs of squeezing rolls can be set in rotation from the same drive shaft with the same transmission ratio. Alternatively or in addition, the rotational speed of the pairs of squeezing rolls can increase in the direction of movement of the plastic film. Then, the diameter of all the pairs of squeezing rolls in the apparatus can be constant. This facilitates stock-keeping.
It goes without saying that the sensitive plastic film and the metal layer deposited thereupon must not be mechanically damaged by the pairs of squeezing rolls. This could be the case if, owing to differing circumferential speed, this slip becomes too great. It is therefore recommendable to provide for each squeezing roll a slip clutch which limits the torque transferred to the lateral surface of the squeezing rolls to a maximum value. The apparatus is then preferably operated such that the slip clutches are always in operation, the slip taking place in the slip clutch and not between the circumferential surface of the squeezing roll and the surface of the plastic film.
The "span", that is the length of the electroplating chamber which can be freely spanned by the plastic film, can be extended, if appropriate, by there being provided inside the chamber, on both sides between the plastic film and a stationary part, a tampon of soft, open-pored plastic foam. The plastic foam is permeable to the electrolyte, that is it does not hinder the electrolysis. In spite of the-softness of the material, the tampon does, however, lend the plastic film a certain stability, so that in particular sporadic yielding or buckling out is made more difficult.
Preferably, the stationary parts are anodes.
Pursuing the same aim is the measure that the holes by which the electrolyte enters into the chamber which can be charged with electrolyte are designed symmetrically on both sides of the plastic film. Then the compressive forces exerted by the inflowing electrolyte on the plastic film compensate one another, so that in turn bending or buckling out in one direction is avoided.
In this case, that design in which the holes are disposed obliquely in such a way that they converge toward one another in the direction of movement of the plastic film proves in turn to be particularly favourable. Thus, the electrolyte is also introduced correspondingly obliquely into the field-filled space of the electrolysis. It has in this case a movement component which is directed parallel to the path of movement of the film and consequently to the film surfaces themselves. On account of the favourable aspect ratio of the through-holes in the plastic film, the lateral surfaces of said holes are nevertheless completely electroplated.
The holes may be formed in the anodes.
As already mentioned above, the spacing of neighbouring bonding devices in the direction of movement of the plastic film is determined substantially by the inner voltage drops in the plastic film. It is so that the conductivity of the plastic film increases during the electroplating operation. Therefore, in an embodiment of the invention, the length of the electroplating chambers can increase in the direction of movement of the plastic film. The respective longitudinal dimensions of the electroplating chambers can be matched to the progressive layer build-up such that, with the same external voltage, it is substantially the same current density that is used everywhere.
However, this aim is not desirable in all cases. Often it may also be preferable for the initial build-up of the metallic layer in the electrolysis initially to take place with lower current densities; the further reinforcing of the metal layer during the course of the electrolysis can then take place with increasing current density. According to one design of the invention, this can be accomplished in a simple manner by all the electroplating chambers in the apparatus having the same length. Since the metallization is already further advanced in the second electroplating chamber, and therefore the conductivity of the plastic film is enhanced, the current density is inevitably greater than in the first electroplating chamber, and so on.
If the current density is to be controlled precisely and, if appropriate, also in adaptation to the plastic film respectively being processed, the potential lying across the anodes of the various electroplating chambers may also differ at least to some extent. This is an additional advantage of the division of the chamber into individual electroplating chambers lying one behind the other.
As already mentioned above, the basic idea of the apparatus according to the invention provides adequate electroplating of a plastic film despite a short residence time in the electrolyte. This result can be enhanced by using an electroplating current source which comprises at least one adjustable pulse generator, the output signals of which are applied to the anode and the bonding device and are square-wave pulses of selectable repetition frequency, clock ratio, amplitude and polarity, the anode being positive on average over time with respect to the bonding device.
It has surprisingly been found that the plating rate can be increased by a multiple if, instead of a constant direct voltage, a pulsed direct voltage is applied at the electrodes of the electrolysis, that is at the anode on the one hand and the plastic film to be electroplated on the other hand. The zero current periods which lie between the individual pulses are compensated by correspondingly increasing the amplitude of the pulses. With the same current consumption, the deposition rate in the case of an apparatus according to the invention, and consequently the current yield, is considerably higher than in the case of the prior art. The physical processes on which this is based have not yet been researched in detail. However, it appears to have been established that a part is played in this by concentration and polarization effects in the region of the anodes and of the plastic films to be plated, which can be favourably influenced in the case of pulsed operation. In particular, the penetration of the metal ions through the double charge layer in the region of the plastic films to be plated should be favoured by the higher voltages which can be employed in the process according to the invention, so that the deposition of metal is facilitated. The precise parameters of the output signals generated by the pulse generator, that is in particular the repetition frequency, the clock ratio and the amplitude, can be optimized by tests and thus adapted to the given geometrical conditions as well as to the particular electrolyte in each case. Different electrolytes, that is in particular different types of metal ions and different additives, may necessitate different types of pulses.
It can be particularly preferred for the electroplating current source to comprise at least two pulse generators which are operated independently of each other and the added output signals of which are applied to the anode and the bonding device, respectively, and the relative phase position of which is adjustable. By the superposing of a plurality of square-wave pulses, in particular two, generated by the independent pulse generators, the characteristic parameters of which pulses can be selected independently of one another, it is possible to compose very differentiated overall pulses, which leads to favourable results.
Particularly fast electroplating rates are accomplished with an embodiment of the invention in which the pulse generator or generators generate such output signals that, during part of the time, the voltage effectively lying across the anode or the bonding device has the reverse polarity, at which the anode is negative with respect to the bonding device. This temporary reversal of the polarity of the operating voltage seems to eliminate in particular disadvantageous concentration effects. It is also possible that this in each case involves a small part of the layer already plated on beforehand going back into solution again, which frees the surface of adhering impurities. In particular, hydrogen embrittlement of the deposited layer is also avoided as a result.
The repetition frequency of the output signals of the pulse generator may be from about 0.1 to about 10,000 Hz.
In many cases, preferential or exclusive electroplating of the lateral surfaces of the through-holes is desired. It has surprisingly been found in the case of the apparatus according to the invention that a preferential deposition of metal takes place on the lateral surfaces of the through-holes if the electrolyte is cooled. Particularly suitable for use is a temperature range from about 10.degree. to about 30.degree. C., preferably from about 18.degree. to about 24.degree. C. Therefore, it can be preferred for the apparatus to include a device with which the electrolyte can be cooled.
Preferably, the apparatus includes a sump for the electrolyte, from which the electrolyte is brought continuously into the chamber which can be charged with electrolyte and into which the electrolyte is returned again from there, and that the cooling device comprises:
By dividing the overall cooling effect between a main cooler and an auxiliary cooler, a particularly precise and rapid automatic control of the electrolyte temperature can be accomplished "locally", that is in the vicinity of the plastic films to be plated. The "main cooling" to the first preselectable temperature takes place already in the sump by a relatively large unit. This first preselectable temperature lies only a little above that temperature which the electrolyte is to reach "locally" The final, second temperature, which lies below the first temperature value, is then effected by the fast-operating auxiliary cooler of lower power, which only influences the electrolyte on its way to the anode.
In previously known apparatus, the plastic films are plated on both sides. Therefore, an electrode respectively extends on both sides of the path of movement of the plastic film. According to a further feature of the invention, in the case of such apparatus it is expediently envisaged that there are provided two auxiliary coolers which can be operated independently of each other, the electrolyte flowing through the first auxiliary cooler being fed to the plastic film on the side facing the one anode and the electrolyte flowing through the other auxiliary cooler being fed to the plastic film on the side facing the other anode.
In the case of one design of this type of the apparatus according to the invention, each auxiliary cooler is assigned a temperature sensor, which is arranged in the vicinity of the plastic film on the side facing the corresponding anode, monitors the local temperature there of the electrolyte and controls the assigned auxiliary cooler accordingly. If there are a plurality of anodes, it may well be expedient for making the application more uniform on the opposite sides of the plastic film to be electroplated to choose the local temperature of the electrolyte to be different, in order to be able in this way to allow for different geometrical conditions, including in the flow movement of the electrolyte.
The anode is expediently an inert dimensionally stable electrode; then there is provided a separate device, by which metal ions extracted during electroplating can be fed back to the electrolyte. The known apparatuses, mentioned at the beginning, use consuming anodes, ie. anode baskets, which are filled with the metal which is to be electroplated on. This metal then goes over into the electrolyte during the electrolysis and thus replaces those metal ions which are lost from the electrolyte due to the deposition on the items to be electroplated. However, inert electrodes, as are proposed according to the invention, result in conditions which can be better reproduced and thus permit more favourable results in plating on. In addition, the downtimes required for servicing can be shortened.
The inert anodes may, for example, be composed of platinized expanded metal or material coated with conductive oxide or of carbon.
If the apparatus according to the invention is used for copper electroplating, the device by which the copper ions extracted during electroplating can be fed back to the electrolyte may comprise:
Metallic copper is not soluble in the copper sulfate solutions in sulphuric acid which are usually used. This changes if the electrolyte is additionally enriched with oxygen. The apportioned oxygen enrichment may thus be used to dissolve chemically a precise amount of metallic copper which is chosen such that the concentration of the copper ions in the electrolyte remains substantially constant.
In this connection there may be provided in particular a pump which takes electrolyte from the sump and feeds it via one or more air injectors to the supply of metallic copper. In this case, the oxygen which is required for dissolving the metallic copper is taken from the ambient air and is admixed with the electrolyte on passing through the air injectors.